Fix custom style on concurrency page applying incorrectly (#4509)

For one, it had an extra `}`, but it was applying to all images on the
page, causing the navbar and other elements to unintentionally be
resized due to extra image padding.

This PR removes the extra `}` while also making the custom style only
apply to elements within the page's article.

Fixes #4508

Author: parlough

src/_guides/language/concurrency/index.md

@@ -6,11 +6,9 @@ description: Use isolates to enable parallel code execution on multiple processo
 <?code-excerpt path-base="concurrency"?>
 
 <style>
-  img {
+  article img {
     padding: 15px 0;
   }
-}
-
 </style>
 
 Dart supports concurrent programming with async-await, isolates, and